Castle Bravo: The largest nuclear device the US ever tested, 1954 [1600x1200]

Before the test, it was expected that the blast would be about 6 megatons, but the actual blast ended up as 15 megatons, and the resulting fallout lead to the largest radioactivity-related accident ever caused by the US.

The fireball was about 7 kilometres across, and the resulting crater was 2 kilometres across. As for the radiation, a graph of the fallout can be seen here, where it reached islands almost 300 miles away from Ground Zero.